1. Field of the Invention
This invention relates to a sub-combustion chamber of an internal combustion engine having a hot plug.
2. Prior Art
Two combustion methods have been used for diesel engines, a kind of internal combustion engines. One method is the direct injection method which directly injects fuel into a single main combustion chamber.
The other method is the sub-combustion chamber method which injects fuel into a precombustion chamber (turbulence chamber) to achieve partial combustion (transient combustion). Recently, diesel engines have become more compact and have had higher speed.
Accordingly, many sub-combustion chamber type engines are used to complete combustion in a short time. However, this kind of a sub-combustion type diesel engine is greatly cooled, since a sub-combustion chamber F is provided in a cylinder head H which has a cooling water passage W as shown by the vertical sectional view in FIG. 1.
As a result, the temperature of compressed air does not rise sufficiently and it is extremely difficult to start the engine especially during a cold season. To solve this problem, a glow plug G is used as a heating means to quicken ignition and to improve starting performance. However, the fuel injected from an injection nozzle N is not completely atomized in the early stage of starting, the area near the sub-combustion chamber F is not sufficiently heated, and the cylinder head H including the sub-combustion chamber and the hot plug P which is a member of the sub-combustion chamber are made of heat-resistant alloys with relatively high thermal conductivity.
As a result of the combination of these factors, the total cooling area is large, turbulence in the sub-combustion chamber F is violent, and thermal loss becomes great. Therefore, a considerably long time is required until the temperature near the sub-combustion chamber rises to the extent that injected fuel does not cause ignition lag. Thus knocking occurs due to the ignition lag, resulting in generation of noise and malfunction of the engine.
To cope with the above-mentioned defects, it is required to quickly raise the temperature of the sub-combustion chamber F. For this purpose, the temperature of a hot plug P in the sub-combustion temperature of a hot plug P in the sub-combustion chamber should be raised quickly, and the thermal conductance from the hot plug P to the cylinder head H which has the cooling water passage W should be as small as possible. The examples of sub-combustion chambers to solve these problems are shown in FIGS. 2 and 3, wherein the outer diameter of the hot plug P is slightly smaller than the diameter of the hot plug installation hole Ha so that a small clearance L for heat insulation is provided between the circumferential surface of the metal hot plug P and the inner wall surface of a hot plug installation hole Ha (except the flange section Pb described below), and the flange section Pb integrated in the lower section of the hot plug P is pressure-fit in the larger diameter section Hb of the hot plug installation hole Ha and is secured in the cylinder head to form the sub-combustion chamber. However, the metallic hot plug P is always exposed to high-temperature flames during engine operation.
Although the plug is generally made of a heat-resistant material, it is not durable enough to cope with the heat. Especially, the edge E of the plug facing the main combustion chamber may be easily burnt.
In addition, the plug is expensive since it is made of a heat-resistant alloy including nickel and cobalt. Considering these situations, various hot plugs made of ceramic materials, whose thermal conductivity is lower than that of metal materials and which have superior heat resistance, have been proposed.
However, ceramic materials are relatively suceptible to thermal and mechanical impacts although they have high heat resistance. When a ceramic hot-plug is used, it cannot be directly pressure-fit into the cylinder head, unlike metallic hot plugs. Moreover, if a ceramic hot plug is installed where a great thermal gradient is caused during engine operation, the ceramic hot plug may be cracked or chipped off due to thermal stress.
Although various proposals have been offered to solve these problems, significantly effective means have not yet been developed.